Heteroatom removal is one of the fundamental processes of the refining and petrochemical industries. Heteroatoms are defined to be those atoms other than hydrogen and carbon, present in hydrocarbon streams, including but not limited to, sulfur, nitrogen, oxygen, and halogens. These atoms are typically found as organo heteroatom molecules wherein the heteroatoms molecules make up part of the carbon hydrogen backbone. Unless otherwise specified, the expression "heteroatom" is hereafter meant to encompass the elemental form of the heteroatom itself as well as its combined counterpart species as an organic and as combined with hydrogen (i.e. organo heteroatom and hetero-hydride, respectively).
The removal of such heteroatoms by conversion to the corresponding hetero-hydride (i.e. hydrogen sulfide, ammonia, water, or hydrogen halide) is typically achieved in industry by reaction of the hydrocarbon stream containing the heteroatoms with hydrogen over a suitable hydroprocessing catalyst which is designed to meet the required product quality specifications, or to supply a low or a substantially reduced level (hereafter low is meant to also include essentially no heteroatoms) heteroatom stream to subsequent heteroatom sensitive processes, catalysts, or product dispositions.
Typically, catalytic heteroatom removal of a stream is carried out in co-current reactors in which both the preheated feed stream and a hydrogen-containing treat gas are introduced to one or more beds of heteroatom removal catalyst. The liquid feed stock, any vaporized hydrocarbons, and hydrogen-containing treat gas all flow together through the catalyst bed(s). The resulting combined vapor phase and liquid phase effluents are normally separated in a series of one or more separator vessels, or drums, downstream of the reactor.
Conventional co-current catalytic heteroatom removal has met with a great deal of commercial success; however, it has limitations. For example, because of hydrogen consumption and treat gas dilution by light reaction products, hydrogen partial pressure decreases between the reactor inlet and outlet. At the same time, any heteroatom hydroprocessing reactions that take place results in increased concentrations of hetero-hydride which strongly inhibits the catalytic activity and performance of most hydroprocessing catalysts through competitive adsorption onto the catalyst. Thus, the downstream portions of catalyst in co-current reactors are often limited in reactivity because of the simultaneous occurrence of multiple negative effects, such as the low H.sub.2 partial pressure and the presence of the high concentrations of hetero-hydride.
Process excursions can occur during operation of a co-current reactor. Process excursions include events such as variation in quality or rate of the liquid feed stream or hydrogen containing treat gas stream, start-up and shut-down of the unit, emergency depressuring of the reactor to avert hazardous conditions, or other process upsets commonly experienced by commercial operating units. During such process excursions, there is a high probability that the heteroatom removal capability of the co-current reactor will be diminished and either the heteroatoms in their original form as organo heteroatom molecules or as the hetero-hydride will come in contact with the heteroatom sensitive downstream process or catalyst. Such contact may cause temporary or permanent impairment of the sensitive process or catalyst and result in unacceptable final product quality which may require significant time and expense (i.e., replacement of a poisoned catalyst) to rectify.
A bed of heteroatom sorbent can be used to protect downstream processes or catalyst but, if a bed of heteroatom sorbent is used downstream of a co-current heteroatom removal zone in co-current operation, a separation step for removal of the hetero-hydride is required. The sorbent bed's capacity can be quickly diminished if substantial heteroatom breakthrough of the upstream heteroatom hydroprocessing catalyst occurs and restoration of capacity will typically require off stream regeneration.
It is relatively well known that heteroatom removal can be accomplished more efficiently in a countercurrent flow hydroprocessing system wherein a hydroprocessing catalyst system through which the liquid hydrocarbon feedstream flows downward and the hydrogen containing treat gas is passed upward. The counter current flow system has the potential to produce significantly lower heteroatom content streams and to do so more efficiently.
While significant potential advantage exist for the application of counter current hydroprocessing; especially when coupled with the use of very high activity heteroatom sensitive catalysts, it is presently of very limited commercial use. U.S. Pat. No. 3,147,210 discloses a two stage process for the hydrofining-hydrogenation of high-boiling range aromatic hydrocarbons. The feed stock is first subjected to catalytic hydrofining, preferably in co-current flow with hydrogen, then subjected to hydrogenation over a heteroatom sensitive noble metal hydrogenation catalyst countercurrent to the flow of a hydrogen containing treat gas. U.S. Pat. No. 3,767,562 and U.S. Pat. No. 3,775,291 disclose a countercurrent process for producing jet fuels, whereas the jet fuel is first hydrodesulfurized in a co-current mode prior to two stage countercurrent hydrogenation. U.S. Pat. No. 5,183,556 also discloses a two stage co-current/countercurrent process for hydrofining and hydrogenating aromatics in a diesel fuel stream.
One reason that countercurrent flow hydroprocessing has not been more widely commercialized is that these type of reactors are more prone to deterioration in performance due to operating excursions than conventional co-current reactor systems. Process excursions include events such as variation in quality or rate of the liquid feed stream or hydrogen containing treat gas stream, start-up and shut-down of the unit, emergency depressuring of the reactor to avert hazardous conditions, or other process upsets commonly experienced by commercial operating units. During said process excursions, there is a high probability that the heteroatom removal capability of the countercurrent reactor will be diminished and either the heteroatoms in their original form as organo heteroatom molecules or as the hetero-hydride will come in contact with the heteroatom sensitive downstream process or catalyst. Said contact may cause temporary or permanent impairment of the sensitive process or catalyst and result in unacceptable final product quality which may require significant time and expense (i.e., replacement of a poisoned catalyst) to rectify.
In light of the above, there is still a need for an improved cocurrent or countercurrent heteroatom removal process, that can reliably operate under commercial plant conditions, to produce streams containing low heteroatom content.